Multiple ram sequence control



2 Sheets-Sheet l ATTORNEYS Oct. 4, 1966 K. E. RAMSEY MULTIPLE EAM SEQUENCE CONTROL Filed May 18, 1964 mw E. E. m E K W FIG] Oct. 4, 1966 K. E. RAMSEY MULTIPLE RAM SEQUENCE CONTROL 2 Sheets-Sheet 2 Filed May 18, 1964 KEITH E. RMSEY FIGI ATTORNEY United States Patent O 3,276,549 MULTIPLE RAM SEQUENCE CONTROL Keith E. Ramsey, Pleasant Lake, Mich., assigner to The Knickerbocker Company, Jackson, Mich., a corporation of Michigan Filed May 18, 1964, Ser. No. 368,031 13 Claims. (Cl. 187-9) The invention pertains to a control system for multiple rams, and particularly relates to a multiple ram sequence control which may be advantageously employed with, but not limited to, hydraulic load elevating devices of the type used on lift trucks or the like.

In many lift truck arrangements wherein a plurality of telescoping mast elements support a load carriage, hydraulic ram means are employed for elevating the load carriage and extending the mast. As the retracted height of a lift truck mast assemfbly is limited by the height of doorways and the like and as it is often desired to elevate the load on the load carriage above ten or twelve feet, it is common to employ multiple ram arrangements to provide the degree of ram extension necessary. Lift trucks employing a free lift wherein the load carriage is raised the height of the retracted mast prior to extension of the mast also often use a multiple ram arrangement.

In a lift truck employing a multiple ram for lifting purposes, problems are encountered in assuring proper sequence of load carriage and mast extension operation during the raising and lowering of the load. For instance, it is usually desired that the first stage of lifting be the elevation of the load carriage without extension of the mast elements. After the load carriage has reached its maximum .position on the retracted mast elements, the mast elements then extend. During retraction of the masts, it is usually desired to maintain the load carriage at its maximum height on the mast elements until the mast elements are fully retracted, and it is at this particular phase of operation that many ram sequencing problems occur. Unless latching or locking means of one type or another are employed, the load carriage often tends to move downwardly upon the extended mast elements as soon as the mast retraction phase begins.

To assure proper sequencing of the load carriage and mast elements during lifting in a free lift arrangement, the effective pressure areas of the various rams are usually .predetermined to provide proper sequencing. Hence, it is usually necessary to manufacture the multiple ram of a rather large diameter if the rams are of the concentrically related type.

It is an object of the invention to provide a multiple ram sequence control wherein proper sequential operation of the rams in a lift truck, or equivalent installation, is positively controlled by means of unique valving devices wherein the need for latches or other mechanical devices to insure the sequencing is eliminated.

A further object of the invention is to provide a multiple ram sequence control which permits the rams to be of smaller diameter than rams of equivalent capacity constructed in a conventional manner. This feature of the invention reduces the material cost of the multiple ram over the cost of equivalent multiple rams of the same capacity, provides more space and clearance within the mast assembly, and significantly reduces the amount of hydraulic fluid necessary to accomplish ram extension as compared with known ram constructions.

Another object of the invention is to provide an automatic multiple ram sequence control which is of an economical and readily manufacturable nature, and which is relatively foolproof in operation.

A further object of the invention is to provide a load- 3,276,549 Patented Oct. 4, 1966 lifting arrangement employing a multiple ram construction wherein a load carriage is supported on telescoping mast elements and the load carriage is capable of being elevated upon the mast elements prior to extension of the elements, and wherein the sequential operation of the load carriage and elements is solely regulated by fluid pressure responsive means.

These and other objects of the invention arising from the details and relationships of the components of an embodiment thereof will be apparent from the following description and accompanying drawings wherein:

FIGS. 1, 2 and 3 are sequential, elevational, schematic views of a lift truck mast assembly during various operational phases, FIG. 1 illustrates the mast'assembly in the fully retracted position, FIG. 2 illustrates the assembly at the end of a free lift wherein the load carriage is raised its maximum extent without extension of the mast elements, and FIG. 3 illustrates the maximum extension of the mast assembly wherein the mast elements are fully extended,

FIG. 4 is an elevational, partly sectioned view of a multiple ram as may be used in the lift truck mast assembly arrangement of FIGS. l, 2, and 3 and employing the sequence control of the invention,

FIG. 5 is an enlarged, sectional diametrical, 4detail View of the multiple ram sequence control during raising of the outer ram wherein pressurized fluid is flowing through the check valve,

FIG. 6 is an elevational, sectional, enlarged, diametrical detail View of the multiple ram sequence control in accord with the invention upon the equalization of the fluid pressures within the rams wherein the check Valve has closed,

FIG. 7 is an elevational, sectional, enlarged, detail view of the sequence control during the lowering of the opter ram upon the inner ram, the section of the sequen-tial control, per se, being taken along section VII- VII of FIG. 8, and

FIG. 8 is a detail, elevational view of the sequence control as taken along section VIII-VIII of FIG. 7.

While the inventive concepts are not limited to use in a multiple ram as employed with a lift truck mast assembly, their use is of advantage in such an environment and will be so described.

FIGS. 1, 2, and 3 illustrate a lift truck mast assembly arrangement in which a multiple ram employing the inventive concepts may be used. This type of mast structure is explained in detail in the assignees United States Patent No. 3,077,951. The front wheel of a lift truck is represented at 10 and the mast structure is mounted on the lift truck frame in any conventional manner. The lift truck mast structure includes an outer mast element 12, an intermediate mast element 14, and an inner mast element 16. The intermediate mast 14 is telescopingly guided within the outer mast 12, and the inner mast 16 is telescopingly supported within the intermediate mast. Each of the masts is preferably formed by a pair of parallels channels opening toward each other. A load carriage 18 is guidably mounted for vertical displacement on the inner mast 16. An inner ram 20 is interposed between the lower portion of the intermediate mast 14 and the upper portion of the inner ram 16. A shorter outer ram 22 for producing the free lift of the load carriage is concentrically mounted on the inner ram 20. A sheave 24 is rotatably mounted on the lower regions of the intermediate mast 14 and a sheave 26 is rotatably mounted on the upper regions of the outer ram 22. A chain 28 is attached to the lift truck frame or outer mast 12 at 30, and extends under the sheave 24, being affixed at 32 to the intermediate mast. A second chain 34 is affixed to the intermediate mast 14 and passes over the sheave 26 and is attached at its other end to the load carriage 18.

When the outer ram 22 is raised on the inner ram 20, the sheave 26 will be raised to lift the load `carriage 18 from the position of FIG. 1 to that of FIG. 2. `It will be noted that during this operation the inner ram does not extend and, thus, no extension of the mast elements takes place. When the load carriage reaches the position shown in FIG. 2, further pressurization of the rams extends the inner arm to elevate the mast elements. Maximum mast element extension is shown in FIG. 3. During retraction of the mast components from the position of FIG. 3, the mast elements fully retract to the position of FIG. 2 before the load carriage 18 lowers from the position of FIG. 2 to that of FIG. 1.

The details of a multiple ram of the type which may be used with the mast structure of FIGS. 1, 2, and 3 employing the concepts of the invention, will be apparent from FIGS. 4 through 8. As shown in FIG. 4, the inner ram 20 consists of a cylinder 36 attached to a lower base portion 38. Passages 40 are defined in the base portion 38 having a threaded communication with a pressurized fluid supply conduit 42 for supplying pressurized uid to the interior of the lcylinder 36. A cylindrical piston element 44 is concentrically related to the cylinder 36 and substantially encompasses the cylinder therein. The cylindrical element 44 is enclosed at its upper region by a head 46 and `a slidable bearing connection between the upper region of the cylinder 36 and the cylindrical element 44 is provided by annular synthetic plastic wear ring 48 mounted on the cylinder. An annular stop ring 50 is also welded to the outer surface of the cylinder 36. The lower portion of the cylindrical element 44 is slidingly and sealingly associated with the cylinder 36 -by an annular adjustable seal member 52. At least one passage 54 is defined in the cylinder 36 whereby pressurized fluid may flo'w into the annular chamber 56 defined between the cylinder and the cylindrical element 44. It will, therefore, be appreciated that pressurized fluid within the cylinder 36 will tend to cause the element 44 to raise with respect to the cylinder to produce extension of the inner ram. 20.

Y The outer ram cylinder 58 is concentrically mounted on the cylindrical element 44 and utilizes the outer surface of the element 44 to define the outer ram chamber 60. An annular sealing ring 62 is aflixed to the outer surface of the element 44 for sealing engagement with the inner surface of the outer cylinder 58. The upper portion of the outer -cylinder 58 is provided with an adjustable sealing gland 64 for establishing a slidable sealed relationship with the outer surface of the cylindrical element 44. A stop ring 66 is aixed to the lower portion of the -outer cylinder 58 for engagement with the lower portion of the seal 62 and, thus, limits the upper movement of the cylinder 58 on the element 44. In a like manner engagement of the seal 52 with the underside of the annular stop ring 50 limits extension of the ele-ment 44 with respect to the cylinder 36.

`In the disclosed embodiment of the invention, the pressurized fluid supplied from the conduit 42 liows from the inner ram 20 to the outer ram 22, and the outer ram receives the iiuid from the inner ram through a sequencing control valve 68 located within a threaded bore 70 defined in the cylindrical element 44. The valve 68 includes an annular body member 72 exteriorly threaded for reception into the bore 70, and an annular seal 74 may be employed to insure a sealing engagement :between the body member and the element. Internally, the body member 72 is provided with a stepped bore 76 defining an lannular radially extending shoulder 78. An annular sealing ring 80 is located within the smaller bore portion for cooperation with the differential pressure valve, as will be described.

An annular differential pressure valve 82 is slidably mounted within the bore of the body member 72. The valve 82 includes an enlarged portion 84 slidingly reeeived within the body member bore and a reduced diameter portion 86 sealingly associating with the seal 80. An annular shoulder 88 is defined on the valve 82 and an annular resilient sealing ring 90 seals the valve with respect to the body member 72.

Internally, the Valve 82 is provided with a cylindrical lbore 92 terminating in a shoulder 94. A conical bore 96 is defined in the valve 82, and .the bore 92 and the conical bore 96 are in concentric communication.

A check valve 98 is located wthin the valve 82 and includes .a head 100 having -a conical surface 102 corresponding to the configuration of the conical bore 96, whereby engagement of these surfaces forms a sealed relationship as shown in FIG. 6. Check valve 98 is -provided with a low stem 104 -terminating in an annular iiange 106, whereby a compression spring 108 may be interposed between the ange and the differential pressure valve shoulder 94 to bias .the valve 82 and the head 100 toward mutual engagement. A plurality of radially extending holes 110 are defined in the stem 104. While the check valve surface 102 is of a true conical configuration, the periphery of the head 100 is preferably square, as will be apparent from FIG. 8. This construction permits fluid communication between :the outer ram chamber `60 and portions of the differential pressure Valve conical surface 96, as will be apparent from FIG. 6.

The area of the valve 82 on portion 84 exposed to the pressure within the inner ram 20 is larger than the `area on surface 96 exposed to the pressure Within the chamber 60, and this relationship is necessary for reasons which will be later described. l

The body member 72 is preferably provided with spanner wrench receiving holes 112, and a bleed valve 114 is located within one of the spanner holes communicating with the bore yof the body member 72 `adjacent the shoulder 78. The bleed valve 114 includes an annular threaded body having a centrally disposed bore receiving a spring 116 and a ball check 118. Thus, it will be apparent that uid tiow through the bleed valve 114 may occur in a direction from the inner ram to the outer ram if fluid leaks past the seals 80 or 90 into the chamber defined between the shoulders 78 and 88.

Operation of the hydraulic sequence control of the invention is as follows:

Assuming the rams 20 and 22 to be fully retracted to the position shown in FIG. 4 wherein the mast elements and load carriage will be at their lowermost positions shown in FIG. l, the components of the control valve 68 will be as shown in FIG. 6 wherein the spring l108 will bias the check valve 98 to the left to engage the corners of the head 100 with the body member 72, and the valve 82 will be biased to the right to maintain an intimate sealing engagement between the conical surfaces 96 and 102. When the pressurized fluid is introduced into the inner ram cylinder 36 through the supply conduit 42, fiuid will flow into the chamber 56 between the cylinder '36 and the cylindrical element 44 and act on the check valve 98. The fluid pressure will bias the check valve 98 to the right, compressing spring 108, whereby pressurized iiuid may flow into the outer ram chamber 60. As the pressurized liuid fills up the system, resistance to movement of the rams will, of course, be due to the weight bearing on the load carriage 18, and that of the mast assembly components.

The effective pressure areas of the inner and outer rams is substantially equal, and as the fluid pressure within both rams will be equal during the lifting operation phase, the router ram 22 will rise on the inner ram 20 to its full extent before the cylindrical element 44 begins to rise with respect to the cylinder 36. The initial movement of the outer ram 22 is due to the fact that less weight is imposed on lthe outer ram than is imposed on the inner ram. The difference in weight primarily constitutes the weight of the inner and intermediate masts.

As the liuid is iiowing into the chamber 60 and the cylinder 58 is rising relative to the inner ram 20. the

valve components will be as shown in FIG. 5. When the cylinder 58 rises sufficiently to raise the load carriage 18 to the position shown in FIG. 2, the seal 62 will engage the stop ring 66. Thereupon, continued introduction of pressurized fluid into the cylinder 36 causes the fluid within the chamber 56 and the chamber 60 to equalize. Under the equalized fluid pressure conditions, the control valve components return to the initial position shown in FIG. 6 wherein fluid flow through the control valve 68 ceases. Of course, the closing of the check valve 98 under equalized fluid conditions is primarily due to the influence of the spring 108. The continued introduction of pressurized fluid into the cylinder 36 causes the element 44 to rise with respect to the cylinder 36 and elevate the mast elements from the position of FIG. 2 to that of FIG. 3. Upon full extension of the mast elements, the seal ring 52 will engage the annular stop ring 50 and the load lifting phase is completed.

To lower the mast assembly components, the operator actuates a suitable valve, not shown, to permit the fluid within the conduit 42 and the cylinder 36 to drain into the hydraulic fluid reservoir. As the pressure within the cylinder 36 is reduced, the element 44 will retract on the cylinder. However, in that back pressure within the cylinder 36, due to the usual throttling orifice used in such hydraulic systems, is still of a significant value, the valve components will maintain the position shown in FIG. 6. When the cylinder element 44 is fully retracted on the cylinder 36 to the position shown in FIG. 4, the fluid pressure within the cylinder 36 will fall to the point wherein the fluid within the outer ram chamber 60, which has been trapped therein during the lowering of the masts from the position of FIG. 3 to the FIG. 2, will act on those portions of the conical bore surface 96 in communication with the outer ram cham'ber 60. As the pressure within the chamber 60 is now considerably larger than that within the chamber 56, the fluid pressure differential acting on valve 82 will cause the valve to slide to the left to the position shown in FIG. 7, permitting fluid to flow into the chamber 56, as `shown by the arrows of FIG. 7. As the fluid passes from the chamber 60, the outer ram cylinder 58 will retract from the position of FIG. 2 to that of FIG. 1 and lower the carriage.

It will, therefore, be appreciated that the differences in the areas at opposite ends of the valve 82 assure a proper sequence of operation of lche rams 20 and 22., and as the fluid is trapped within the outer ram chamber `6) during the lowering phase while the `masts are extended, a fluid lock is present which assures proper sequential operation of the rams. Should fluid leak past the seals 80 or 90 into the chamber between the shoulders 78 and 88, the bleed valve 114 will permit the fluid to escape from this chamber under the influence of the spring 108.

While the inventive concepts of the multiple ram sequence control have been illustrated in a concentric ram relationship, it will be appreciated that the inventive principles may be employed with a pair of rams which are not of a concentric construction, and the control valve may take a number of various forms. It is, therefore, the intention that although those skilled in the art may arrive at other forms of the invention than that shown, but within the spirit and scope thereof, that the invention be defined only by the following claims.

I claim:

`1. A hydraulic ram arrangement comprising, in cornbination,

(a) a first ram including cooperating piston and cylinder elements,

(b) pressurized fluid supply means communicating with said first ram,

(c) a second ram including cooperating piston and cylinder elements,

(d) unidirectional flow control valve means interconnecting said first and second rams permitting fluid flow from said first -ram to second ram, and

(e) differential pressure-operated valve means interconnecting said first and second rams normally preventing fluid flow from said second ram. to said first ram, but permitting fluid flow from said second ram to said 5 first ram upon the fluid pressure within said first ram being reduced a predetermined degree below the fluid pressure within said second ram.

2. A hydraulic ram arrangement comprising, in combination,

(a) a first ram including cooperating piston and cylinder elements,

(b) pressurized fluid supply means communicating with said first ram,

(c) a second ram including cooperating piston and cylinder elements,

(d) fluid passage means establishing communication between said first and second rams, and

(e) avalve mounted within said passage means controlling fluid yflow therethrough, said valve including first means .permitting fluid flow from said first ram to said second ram and preventing fluid flow from said second ram to said first ram and second means permitting fluid flow lfrom said second ram to said first ram only upon the fluid pressure within said first ram becoming less than that of said second ram by a predetermined value.

3. A hydraulic ram arrangement comprising, in combination,

(a) a first ram including cooperating piston and cylinder elements,

(b) pressurized fluid supply means communicating with said first ram,

(c) a second ram including cooperating piston and cylinder elements,

(d) fluid passage means establishing communication between said first and second rams, and

(e) a valve mounted within said passage means controlling fluid flow therethrough, said valve including check valve means permitting fluid flow from said first ram to said second ram and pressure differential operated means having opposed `operative surfaces exposed to the fluid pressure within said first and second rams, said pressure differential operated means permitting fluid flow from said second ram to said first ram only upon the fluid pressure within said first ram becoming less than that of said second ram by a predetermined value.

4. In a hydraulic ram arrangement for a lift truck hav- 50 ing load lifting elements having first and second stages of raising and lowering movement, in combination,

(a) a first ram connected to said load lifting elements adapted to produce said second stage of movement during raising and said first stage of movement during lowering,

(b) pressurized 'fluid supply means communicating with said first ram.

(c) a second ram connected to said load lifting elements adapted to produce said first stage of movement during raising and said second stage of movement during lowering,

(d) unidirectional flow control valve means interconnecting said first and second rams. permitting fluid flow from said first ram to said second ram during said first stage of movement during raising, and

(e) differential pressure-operated valve means interconnecting said first and second rams normally preventing fluid flow from said second ram4 to said first ram and permitting fluid flow from said second ram to -said first ram only upon the fluid pressure within said first ram being reduced a predetermined degree below the fluid pressure within said second ram to produce said second stage of movement during lowermg.

`5. In a hydraulic ram arrangement for a lift truck having load lifting elements having first and second stages of raising and lowering movement, in combination,

(a) a first ram connected to said load lifting elements adapted to produce said second stage of movement `during raising and said rst stage of movement during lowering,

(b) pressurized fluid supply means communicating with said first ram,

(c) a second ram connected to said load lifting elements adapted to produce said first stage of movement during raising and said second stage of movement during lowering,

(d) fluid passage means establishing communication between said first and second rams, and

(e) a valve mounted within said passage means controlling fluid flow therethrough, said valve including check valve means permitting fluid flow from said first ram to said second ram to produce said first stage of movement during raising and pressure differential operated means having opposed operative v surfaces exposed to the fluid pressure within said first and second rams, said pressure differential operated means permitting fluid flow from said second ram to said first ram during the second stage of the lowering movement only upon the fluid pressure within said first ram becoming less than that of said second ram by a predetermined degree.

6. A hydraulic ram arrangement in a lift truck having extensible mast elements and a load carriage mounted on said mast elements for vertical movement thereon cornprising, in combination,

(a) a first ram operatively connected to said mast elements adapted to extend and retract said elements,

(b) pressurized fluid supply means communicating with said first ram,

(c) a second ram operatively connected to said load carriage adapted to raise and lower said carriage upon said mast elements,

(d) unidirectional flow control valve means interconnecting said first and second rams permitting fluid flow from said first ram to said second ram to raise said load carriage upon said mast elements prior to extension of said mast elements, and

(e) differential pressure-operated valve means interconnecting said first and second rams normally preventing fluid flow from said second ram to sai-d first ram and permitting fluid flow from said second ram t-o said first ram only upon the fluid pressure within said first ram being reduced a predetermined degree below the fluid pressure within said second ram, lwhereby said second ram lowers said loa'd carriage on said mast elements only after said elements have been retracted.

7. In a hydraulic ram arrangement as in claim 6 wherein:

(a) said second ram is concentrically mounted on said first ram and rises upon extension of said mast elements, the effective pressure area of said second ram ybeing so related to the effective pressure area of said first ram that said second ram extends and raises said load carriage prior to extension of said first ram.

8. A hydraulic ram arrangement in a lif-t truck having an extensible mast element and a load carriage mounted upon said extensible mast element for vertical movement thereon comprising, in combination,

(a) a first ram connected to said extensible mast element adapted to extend and retract said element,

(b) pressurized fluid supply means communicating with said first ram,

(c) a second ram mounted on said extensible mast element operatively connected to said load carriage adapted to raise and lower said load carriage on said extensible mast element,

(d) said rams having effective pressure areas so related that equal fluid pressures within said rams extend said second ram and raise said load carriage prior to extending of said first ram,

(e) unidirectional flow control valve means interconnecting said first and second rams permitting fluid flow from said lfirst ram to said second ram to raise said load carriage upon said mast elements prior to extension of said mast elements, and

(f) differential pressure-operated valve means interconnecting said first and second rams normally preventing fluid flow from said second ram to said first ram and permitting fluid flow from said second ram to said first ram only upon the fluid pressure within said first ram being reduced a predetermined degree below the fluid pressure within said second ram, whereby said second ram lowers said load carriage on said mast elements only after said elements have been retracted.

9. A hydraulic ram arrangement in a lift truck having an extensible mast element and a load carria'ge mounted upon said extensible mast element for vertical movement thereon comprising, in combination,

(a) a first ram connected to said extensible mast element adapted to extend and retract said element, (b) pressurized fluid supply means communicating with said first ram,

(c) a second ram mounted on said extensible mast element operatively connected to said load carriage adapted to raise and lower said load carriage on said extensible mast element,

(d) fluid passage means interconnecting said first and second rams,

(e) said rams having effective pressure areas so related that equal fluid pressures within said rams extend said second ram and raise said load carriage prior to extending of said first ram, and

(f) a valve Within said fluid passage means, said valve including check valve means permitting fluid flow from said first ram to said second ram to extend said second ram and raise said load carriage on said extensible mast element, and pressure differential operated means having opposed operative surfaces exposed to the fluid pressure within said rst and second rams, said pressure differential operated means permitting fluid flow from said second ram to said first ram only upon the fluid pressure within said first ram becoming less than that of said second ram by a predetermined degree to retract said second ram and lower said load carriage on said extensible mast element.

10. A ylift truck assembly comprising, in combination,

(a) an elevatable mast element mounted upon support structure,

(b) a load carriage mounted upon said mast element Ifor vertical movement thereon,

(c) a first ram connected to said extensible mast element adapted to extend and retract said element, (d) pressurized fluid supply means communicating with said first ram,

(e) a second ram mounted on said extensible mast element operatively connected to said load carriage adapted to raise and lower said load carriage on said extensible mast element,

(f) said rams having effective pressure areas so related that equal fluid pressures Within said rams extend said second ram and raise said load carriage prior to extending of said first ram,

(g) fluid passage means establishing communication between said first and second rams,

(h)unidirectional flow lcontrol valve means located in said fluid passage means permitting fluid flow from said first ram to said second ram to raise said load carriage upon said mast elements prior to extension fof said mast elements, and

(i) differential pressure operated valve means located in said fluid passage means permitting uid flow from said second ram to said rst ram only upon the fluid pressure within said first ram being reduced a predetermined degree below the fluid pressure within said second ram, whereby said second ram lowers said load carriage on said mast elements only after said elements have been retracted.

11. A lift truck assembly comprising, in combination,

(a) an elevatable mast element mounted upon support structure,

(b) a load carriage mounted upon said mast element for vertical movement thereon,

(c) a first ram connected to said extensible mast element adapted to extend and retract said element, (d) pressurized fluid supply means communicating with said first ram,

(e) a second ram mounted on said extensible mast element operatively connected to said load carriage adapted to raise and lower said load carriage on said extensible mast element,

(f) fluid passage means interconnecting said rst and second rams,

(g) said rams having effective pressure areas so related that equal fluid pressures `within said rams extend said second ram and raise said load carriage prior to extending of said iirst ram, and

(h) a Valve `within said fluid pass-age means including check valve means permitting Huid ow from said rst ram to said second ram to extend said second rarn and raise said load carriage on said elevatable mast element and pressure differential operated means having opposed operative surfaces exposed to the Huid pressure within said first yand second rams, said pressure differential operated means permitting fluid ow from said second ram to said rst ram only upon the liuid pressure within said rst ram becoming less than that of said second ram by a predetermined degree to retract said second ram and lower said load carriage on said extensible mast element.

12. In a lift truck assembly as in claim 11 wherein:

(a) said second ram is concentrically mounted on said rst ram and rises during the extension of said first `ram and raising of said extensible mast element.

13. In a lift truck assembly `as in claim 11 wherein:

(a) said valve includes an annular body member,

(b) said pressure differential operated means comprising an annular piston having opposite ends slidably mounted within said body member,` said opposed surfaces being defined on said opposite ends,

(c) said check valve means being mounted within said piston and having a head adapted to selectively sealingly engage one of said ends of said piston, and

(d) a spring interposed between said piston and said check valve means biasing said check valve head and the adjacent piston opposite end t-oward mutual engagernent.

References Cited by the Examiner UNITED STATES PATENTS 2,676,573 4/1954 Abbe 91-169 3,067,770 12/1962 Faucher 137-493 3,077,951 2/1963 Ramsey 187-9 EVON C. BLUNK, Primary Examiner.

HUGO O. SCHULZ, Examiner.

H. C. HORNSBY, A ssistrmt Examiner. 

1. A HYDRAULIC RAM ARRANGEMENT COMPRISING, IN COMBINATION, (A) A FIRST RAM INCLUDING COOPERATING PISTON AND CYLINDER ELEMENTS, (B) PRESSURIZED FLUID SUPPLY MEANS COMMUNICATING WITH SAID FIRST RAM, (C) A SECOND RAM INCLUDING COOPERATING PISTON AND CYLINDER ELEMENTS, 